When drilling a wellbore, maintaining a vertical drilling direction may be desired. However, slight deflections of the bottom-hole assembly (BHA) drill string may cause the wellbore to deviate from the vertical axis and thus the wellbore may not propagate as planned. Vertical control devices may be utilized to correct deviation from vertical. Likewise, steerable systems may be utilized to control the direction of propagation of the wellbore. Typically, these devices may include a rotating section, including the drill bit and any associated shafts, and a non-rotating section which remains substantially non-rotating relative to the surrounding formation.
Steerable drilling systems are often classified as either “point-the-bit” or “push-the-bit” systems. In point-the-bit systems, the rotational axis of the drill bit is deviated from the longitudinal axis of the drill string generally in the direction of the wellbore. The wellbore may be propagated in accordance with a three-point geometry defined by upper and lower points of contact between the drill string and the wellbore, defined as touch points, and the drill bit. The angle of deviation of the drill bit axis, coupled with the distance between the drill bit and the lower touch point, results in a non-collinear condition that generates a curved wellbore as the drill bit progresses through the formation.
In push-the-bit systems, a non-collinear condition may be achieved by causing one or both of upper and lower stabilizers, for example via blades or pistons, to apply an eccentric force or displacement to the BHA to move the drill bit in the desired path. Steering may be achieved by creating a non-collinear condition between the drill bit and at least two other touch points, such as upper and lower stabilizers, for example.